The pattern and realization of zigbee wi-fi wireless pathway

IJRET: International Journal of Research in Engineering and Technology ISSN: 2319-1163
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Volume: 01 Issue: 03 | Nov-2012, Available @ http://www.ijret.org 373
THE PATTERN AND REALIZATION OF ZIGBEE -WI-FI
WIRELESS PATHWAY
P.Sasi Kala1
, K.Nishakar2
1
Assistant Professor, Department Of ECE, Abhinav Hi-tech College Of Engg, Hyderabad, India.kala.smile@gmail.com
2
Associate Professor, Department Of ECE, Vidhya Vikas Institute of Technology, Hyderabad, India
Abstract
The application of WSN/ZigBee is having enormous growth and how to connect WSN/ZigBee to the present standard network
seamlessly is an issue what is worth studying. In this paper, it patters and realizes a ZigBee—Wi-Fi wireless pathway based on
STM32W108 RF chip and embedded Wi-Fi module. In ZigBee network, wireless pathway as a sink, it receives information from
sensor nodes and interacts with them. In WLAN, wireless pathway communicates with PC or network servers by means of AP. Both
the hardware scheme and software scheme of the wireless gateway are introduced. Then the performance of the wireless pathway is
tested, and the result shows that it can be used for general purposes and the performance is stable. The wireless pathway can realize
communication effectively between ZigBee network and WLAN.
Index Terms: STM32W108, ZigBee, WI-Fi, Wireless gateway
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1. INTRODUCTION
A wireless sensor network (WSN) consists of a number of
distributed devices using sensors to cooperatively monitor
physical or environmental conditions, such as temperature,
sound, vibration or pressure at different locations. ZigBee is a
new IEEE802.15.4 standards-based, short distance, low-data
rate and low-power consumption wireless communication
technology. Moreover, ZigBee specification has better
compatibility between versions from earliest ZigBee 1.0, ZigBee
1.1 to ZigBee 2007/PRO. With the constantly improving and
maturing of ZigBee technology, it is widely used in WSN. Wi-Fi
is a most successful wireless local area network (WLAN) system
that builds upon the IEEE 802.11 standards. With the rapid
development of Wi-Fi in recent years, infrastructure facilities
have been improved. What’s more, the coverage of wireless
access points (AP) has already been very wide and the price is
cheap. Usually, the bandwidth of WLAN is higher than the
bandwidth of other types of internet connection such as ADSL,
GPRS and 3G and the transmission delay of WLAN is less than
theirs. In this paper, a small in size, low-power, low-price and
lightweight ZigBee–Wi-Fi wireless gateway is introduced. It
contains two functions. For one thing, in wireless sensor
network based on ZigBee (it is called ZigBee network for short),
terminal sensor nodes and route sensor nodes are responsible for
collecting and processing data. They will employ ZigBee
technology to communicate with wireless pathway. The wireless
gateway encapsulates the data that received from ZigBee
network according to a certain format and transmits them to Wi-
Fi module. For another, Wi-Fi module sends data to monitoring
software on PC by means of AP. In addition, the Wi-Fi module
sends commands from PC to ZigBee network. Fig. 1 displays
the system operation model.
2. MODEL OF HARDWARE
The hardware platform of wireless pathway is made up of two
parts: the RF chip STM32W108 that integrates a 2.4 GHz, IEEE
802.15.4-compliant transceiver and 32-bit ARM® CortexTM-
M3 microprocessor and EMW-380 Wi-Fi module. The structure
is demonstrated in Fig. 2.
Fig-1.communication model between Zig Bee and Network

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Fig-2.Hardware of Wireless Gateway
2.1. STM32W108
STM32W108 is a fully integrated System-on-Chip (SOC)
launched by STMicroelectronics recently. It has 128 Kbytes of
embedded Flash memory, 8Kbytes of integrated RAM memory
for data and program storage and peripherals (like USART, SPI,
TWI, ADC, general-purpose timers and 24 GPIOs) of use to
designers of ZigBee-based systems. The processor can be
operated at 12 MHz or 24 MHz when using the crystal
oscillator, or at 6 MHz or 12 MHz when using the integrated
high frequency RC oscillator. The difference between
STM32W108 and other 2.4GHz SOC chips are as follows:
(1) STM32W108 adopts 32-bit ARM® Cortex -M3
processor, which can improve processing performance based on
lower power consumption.
(2) It supports for external power amplifier and the output
power is up to +7dBm.
(3) Different types of STM32W108 solidify different protocol
stacks such as 802.15.4 MAC Ember Net ZigBee PRO and
ZigBee RF4CE Users do not have to understand the network
protocol development and they can directly exploit wireless
networking products that comply with the relevant standards,
which can greatly simplify the development of the technical
complexity products and shorten time to market. In this paper,
the type of STM32W108 used integrates Ember Net ZigBee
PRO Stack. Meanwhile, STM32W108 is responsible for
wireless communication between wireless gateway and ZigBee
sensor nodes.
2.2. EMW-380 Wi-Fi Module
The wireless gateway adopts EMW-380 Wi-Fi module to realize
the WLAN capabilities. It is an embedded Wi-Fi (802.11b/g)
applicable module. The hardware is composed of ARM
processor and Wi-Fi RF chip. The software integrates some
network protocols such as Wi-Fi, TCP/IP, UDP, and DHCP.
The module provides an SPI/UART interface to connect with
MCU. A simple API command set is provided to implement link
layer data services based on 802.3 frame formats. The module
supports AP and Ad-Hoc, RF channel automatic choice and
WEP encryption, which is suitable for small system with
standard Wi-Fi access.
3. SOFTWARE DESIGN
The software architecture of the wireless gateway includes
System control, software design of EMW-380 Wi-Fi module,
software design of STM32W108 and wireless gateway
application layer protocol. The software architecture is shown in
Fig.3 System control regulates hardware and application layer
protocol of the wireless gateway.
Fig3. Software Structure of Wireless gateway
Table1. EMSP Protocol Format
3.1 EMW-380 Wi-Fi Module Design
EMW-380 Wi-Fi module communicates with STM32W108
through UART and EMSP protocol. And the EMSP protocol
has nothing to do with the physical connection. The module has
two operative modes: configuration mode and data transparent
transmission mode. In the configuration mode, all the work is
controlled by EMSP command. In the data transparent
transmission mode, module encapsulates the received data into
TCP/UDP packets and sends them to remote end. Furthermore,
it can send the TCP/UDP packets that come from remote end to
STM32W108. The data packet format of EMSP protocol is
shown in Table1. It contains a protocol head (8 Bytes) and data
field (maximum is 256 Bytes).As shown in Table 1, all the
request and response commands are checked to ensure the
integrality and reliability. The EMSP protocol consists of 12
commands to implement module control, network control and
network communication.
3.2. STM32W108 Design

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From the point of ZigBee network, the wireless gateway is a
sink and gathers all kinds of data from sensor nodes. The Ember
Net ZigBee PRO provides users with a standard networking API
based on the ZigBee specification across the STM32W108
platforms. Users can just learn how to use API functions on the
application layer, and then can develop own projects. Due to
increasing flexibility and reliability of EmberZNet ZigBee PRO
Stack, it supports three types of mesh network topologies. star
network, full mesh network and hybrid mesh network. Aiming at
the application design of the wireless gateway, hybrid mesh
network is used. The stack mentioned above allows users to
create own wireless personal area network (WPAN). The basic
functions of stack contain network organization, route
discovery, routing, device discovery, message relay and
security. Under the application layer of protocol stack is
followed by transport layer, application support layer, and
network Routing and discovery, MAC and RF abstraction layer.
Physical RF and medium access provide hardware support for
software design.
3.3. Application Layer Protocol of The Wireless
Gateway Design
The application layer of wireless gateway software design is
described in Fig. 4.
Fig.4-Application layer protocol wireless gateway software
design
When initializing STM32W108,INTERRUPTS_ON() And
halInit() are used to configure clock, open system Timer, and
turn on interruptions.EM380C_fast_Init(BAUDRATE) provided
by EMSP protocol API set can configure the interface between
STM32W108 and EMW-380 Wi-Fi module. It will take about
1-2 seconds to completely initialize the module, and then it will
respond to the request sent by STM32W108. STM32W108 can
query the /INT pin of EMW-380 Wi-Fi module to determine
whether the initialization is complete. After finishing
initialization, /INT changes fromhigh to
low.EM380C_Set_Config(&parms) can set the configuration
parameters of the module. And it will launch the module when
using EM380C_Startup() function. Once the module has
connected to monitoring software on PC as a client, it will enter
into transparent data transfer operative mode. Then the module
can send the data received from STM32W108 to monitoring
software. Besides, the module will send the commands from
monitoring software to STM32W108.
After finishing the work of EMW-380 Wi-Fi module, calling
ember nit() can complete the initialization of Ember Net ZigBee
PRO Stack and RF module. The network initialization is done
by emberNetworkInit(). Furthermore, the address table also
should be initialized. After entering into event loop,
emberFormAndJoinTick () function can form network according
to the given network parameters and join in it. And then the
permit joining flag becomes TRUE. The application Tick()
function offers some services: (1) checking timeout, (2) sending
a route request packet that creates routes from every node in
ZigBee network back to wireless gateway, (3) calling sink
Advertise() to send multicast message, (4) detecting whether it
have received data from EMW-380 Wi-Fi, (5) updating the
address table, (6) setting the time permitted join in network.
Of course, the wireless gateway plays an important role in the
communication model between ZigBee network and WLAN. On
the side of ZigBee network, the wireless gateway takes charge of
processing data on the application layer. NWK and MAC layers
are managed by EmberZNet ZigBee PRO Stack while
STM32W108 handles with PHY layer. From the point of view
of WLAN, wireless gateway also deals with data on the
application layer and other layers like TCP/IP, 802.11 MAC and
802.11 PHY are regulated by EMW-380 Wi-Fi module. There
are two directions of data communication in communication
model between ZigBee network and WLAN.
3.4 Data from Zigbee Network to Wlan
In the ZigBee network, after establishing a network by wireless
gateway, sensor nodes join in the network within the time that
wireless gateway permits. When the time expires, sensor nodes
can’t join in the network anymore unless the button is pressed
on route sensor nodes and then the permit joining flag becomes
TRUE again. But if the sensor node is the terminal node, it can’t
allow other nodes to join in the network through itself. So the
size of the network can be dynamically changed, and the
extensibility of ZigBee network is good. Every node has a

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unique NodeID assigned by wireless gateway once join process
has completed. After interacting with wireless gateway as shown
in Fig.5,sensor nodes begin to send data to wireless gateway.
SINK_ADVERTISE is sent regularly, and the time interval can
be set by users. And they will record each other’s MAC address
and NodeID in their address tables before exchanging data. This
interaction procedure can ensure that the data that sensor nodes
send to wireless gateway is correct and integral. When the
sensor node is a sleep node, it will enter into power save mode
once it is idle. If the sleep node misses the SINK_ADVERTISE
sent by wireless gateway, it can send SINK_QUERY message to
wireless gateway asking for SINK_ADVERTISE and then start
to send data to wireless gateway after finishing the interaction
procedure in Fig. 5.
When wireless gateway receives data from ZigBee network, it
will parse data in terms of the type of monitoring data in
application layer. And then it again packages the monitoring
data according to the format in Table 2. The start fields of data
from ZigBee network to WLAN are 0x7E and 0x42 and the end
field is 0x7E. The Payload data contain MSG_HEADER and
MSG_DATA. MSG_HEADER consists of the length of
MSG_DATA, MAC frame control field, destination PAN
identifier, destination address, data type (data, command, data
response or command response) and the group information of
sensor nodes. MSG_DATA includes NodeID and monitoring
data while monitoring data will vary with different applications.
The check field is CCS checksum, which is used to judge
whether the data are correct. TCP/IP protocol embedded within
EMW-380 Wi-Fi module encapsulates the data that are from
application layer of wireless gateway. Then in transparent
transmission mode, it sends the encapsulated data to WLAN.
The monitoring software processes the received data and
feedback to the users.
Fig-5.Communication procedure between sensor node and
wireless gateway
3.5 Data from Wlan to Zigbee Network
The way of transmitting data from WLAN to ZigBee network is
almost the same. The data from the WLAN is first transmitted to
EMW-380 Wi-Fi module. It extracts the data from the TCP
packet, and transmits them to the application layer of wireless
gateway. The application layer of wireless gateway parses the
data according to the format in Table 2, and hands over the data
to Ember Net ZigBee PRO Stack. The stack will assemble them
to IEEE 802.15.4 frame and send to destination node in ZigBee
network. The process includes the routing of the network, so
that the data sent to the specified node by the WLAN always
reach the destination correctly. Besides, the node is identified by
the NodeID. When the node receives data, then it handles with
them further.
4. RESULTS
In this section, result are discussed. Hardware test environment
includes wireless gateway, TP-LINK TL-WN322G+ 54Mbps
wireless USB network card, PC, several sensor nodes, network
sniffer produced by Mxchip Company and a network protocol
analyzer for UNIX and Windows called Wire shark.
4.1 Functional Test
Firstly, the function of wireless gateway is tested. As shown in
Fig. 6, the monitoring software on PC is a server and its IP
address is 192.168.4.7. While the IP address of wireless
gateway is 192.168.4.53 and it is a client. While the Wire shark
software can capture the data packet that wireless gateway sends
to monitoring software.
Fig.6- information shown by monitoring software
For example, the data that wireless gateway send to monitoring
software are 7E 42 0C 00 00 2B 00 FF 00 FF 11 00 71 67 00 00
00 00 FF FF FF FF FF FF CD 00 00 00 58 A7 7E. Among the
31 Bytes data, CD and 00 are just light intensity value 0x00CD
measured by sensor node. Its NodeID is 26481. Comparing
0x00CD with the data of ZigBee network layer packet that
captured by network sniffer, the result is that they are same. The
decimal number of 0x00CD is 205 and it can be seen from Fig.
6.
4.2 Performance Test
1). Test scenario 1
Sensor nodes distribute randomly around the wireless gateway.
The rule is that each node sends only 1000 data packets to
wireless gateway and then stop. The length of each packet is 82
Bytes, which is the maximum packet load length that users used

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on application layer. As Fig. 7 described, x-axis denotes the
number of nodes in ZigBee network and y-axis represents the
average packet loss rate (PLR) of wireless gateway. The four
curves respectively show the change of PLR when the time
interval of sending data is 80ms (millisecond), 100ms, 120ms or
200ms.mIn the above four cases, ten tests are done
respectively. So the PLR is an average value of the ten tests. As
the time interval increases, PLR drops. If the time interval of
sending a data packet is 1 second and there are 8 sensor nodes in
the ZigBee network, the PLR of wireless gateway drops to 0%.
The reason why PLR varies with the time interval of sending
data packet is that PLR depends on three factors: the number of
sensor nodes (N) in ZigBee network, the time interval of
sending data packet (T) and the length of data packet (L). When
N and L remain the same and T gets smaller, the number of
packet sent to the air per second increases, and then the collision
probability (P) raises when sending data packet. Although there
is data
Retransmission mechanism in EmberZNet ZigBee RPO Stack
and retransmission number is up to 3, the consecutive collision
probability that is P3 would be substantially increased.
Therefore, when time interval grows, P gets smaller and PLR
falls dramatically. So T is an important influential factor to PLR.
Fig 7.Data loss rate changes when different time interval used
Fig-8 Data loss rate changes when the length of packet varies
CONCLUSIONS
In this paper, the ZigBee—Wi-Fi wireless pathway based on
STM32W108 RF chip and EMW-380 Wi-Fi module can connt
the ZigBee network to standard network seamlessly. From the
result of performance test we can see that the performance and
stability of wireless gateway suits the usual target of WSN
application, which is low real time demand, small amount of
data transmission and low bandwidth. As applications expansion
of WSN, further research can focus on: the low power design of
wireless gateway, using wireless gateway ID to identify different
deployment environments and adopting embedded Web Server
technology enables users to visit different WSNs information.
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